In China, The exploration of rules governing the fair use of personal information began with commercial innovation, progressed through judicial practice, and was eventually expressed in law, as it is a necessary requirement of the nature of information flow and social attributes. However, due to the legal constraints of the “personal attribute” of personal information, there is no consistent definition of the legal nature of fair use behavior of information, and case judgments also face difficulties in determining the reasonableness of information processing behavior. These issues involve the normative construction of fair use of personal information, which has been studied by Chinese scholars, but has not been deeply explored in specific contexts. This article starts with the legal theory transformation of information regulation in the data society, attempting to clarify the legal nature of fair use of personal information, summarize the factors for determining fair use, and explore the judicial application logic of reasonableness in specific contexts. Based on Chinese judicial practice and comparative law experience, fair use of personal information can be defined as another legitimate basis for information processing outside the “informed consent” principle. Unlike the nature of “exclusion of liability” and “restrictions on rights”, fair use under the concept of data sharing should be defined as a basic competence of personal information, which can be based on the maintenance of public interests and protection of individual rights, and does not exclude reasonable commercial use. Fair use of personal information should satisfy three elements: reasonable purpose, reasonable method, and no unreasonable harm to the information subject. Using commercial fair use, which is frequently disputed in practice, as an example, based on considerations of economic benefits and institutional evolution, the processing of personal information by commercial subjects is legitimate. However, due to the personal attribute of the information, the processing behavior in this context should be subject to the limits of low sensitivity of the information and strict disclosure management obligations of the merchants, and the reasonableness of the behavior should be determined based on the nature of the specific behavior, with the inherent nature of the information being secondary. This paper contributes to the existing literatures in two aspects: first, based on the demands of ordered sharing of personal information, it explores the legitimacy of fair use from the perspective of information processors, defines fair use of personal information as a “basic competence,” and better leverages the value of information property. Second, in addition to “reasonable purpose” and “reasonable method,” this article includes “no unreasonable harm to the information subject” as a factor in determining the reasonableness of processing behavior. At the same time, based on practice, it explains the legitimate basis and steps, elements for determining the reasonableness of commercial fair use in specific scenarios, and proposes security management obligations for commercial subjects. Our research starts from the perspective of information processors, to some extent, defining the boundary of information processing behavior, which helps commercial subjects conduct data business in an orderly manner and avoid this rule becoming a legal tool for information control subjects to avoid responsibility. It is also hoped that it can provide rules and logic for determining the behavior of “fair use” of personal information in judicial practice. [ABSTRACT FROM AUTHOR]